1. Field of the Invention
The invention relates to imaging devices, systems and methods, and in particular to a semiconductor pixel imaging device for use as an image sensor and to imaging systems and methods utilizing the pixel semiconductor imaging device.
2. Description of the Prior Art
Two basic types of semiconductor pixel devices are known in the prior art:
1) Charge Coupled image sensors also known as Charge Coupled Devices (CCD) and
2) Pulse Counting Semiconductor Pixel Devices.
CCDs have been used for the past 15 years or so (see for example S. M Sze “Physics of Semiconductor Devices” 2nd Edition, 1981) as image sensors. Practically all CCDs available are made using silicon (Si) technology. The principle of operation of a CCD is based on the fact that when an appropriate voltage is applied via an electrode gate, the bulk Si volume becomes depleted of majority carriers (e.g. holes) and a region is created (depletion region) where electrons can be accumulated. This depletion region amounts to a potential well with a depth proportional to the applied voltage. The maximum charge that can then be stored in a CCD pixel depends on the area under the electrode, the voltage applied, the dark or leakage current coming from the bulk Si that continuously fills the well and the thickness of the oxide layer between the electrode and the bulk Si. These factors determine the effective CCD charge storing capacity.
When electrons are accumulated in the potential well and need to be read out, the potential at the electrode gates is pulsed and an electron package stored under one gate starts to be clocked towards the next gate and so on. The electron package never leaves the Si substrate and in order to read a stored charge at some pixel position the contents of all other pixels ahead of it have first to be read out in a sequential way. During this process no further charge can be accumulated as it would destroy the information of the charge content per pixel and consequently it would spoil image resolution and contrast. Therefore during readout the image sensor is inactive. The above described process requires at least three electrode gates per pixel.
CCDs can be used either for detecting, accumulating and reading out charge created from light and/or radiation or can be used just as a readout device for reading the charge created in another detecting means (e.g. photodiodes). When used for detecting incident radiation as well as for reading the signals, CCDs have an additional limitation of low efficiency.
In particular at high energies (X-rays above a few KeV) CCDs are used in conjunction with light converting screens that convert X-rays to optical light, to which a CCD is more sensitive. However light diffusion worsens resolution and contrast.
Therefore a CCD operates in the following way:
1) Charge is accumulated within a depletion region created by an applied voltage. For each pixel the depletion region has a potential well shape and constrains the electrons under the electrode gate to remain inside the Si bulk volume.
2) Voltages are pulsed to the electrode gates to clock each charge package to the volume corresponding to the next pixel. The charge package remains at all times inside the Si substrate and clocks its way through, pixel by pixel, to a common output. During that process additional charge cannot be accumulated.
As a result of the above the CCD is a device with two substantial limitations:
1) Compromised dynamic range. Typically a CCD can accumulate 100,000-700,000 electrons. The reason for the limited dynamic range is that the potential well fills up due to the dark current within the Si volume, the electrode gate surface under which the charge is accumulated is at best ⅓ of the total pixel area (thus not utilizing the total charge storage capacity of the pixel) and the oxide layer thickness upon which the storage capacity also depends has to be thick to stand the abrupt voltage pulses needed for the readout (note: the thicker the oxide layer, the less charge can be stored in the potential well).
2) Large inactive time. The inactive time needed for the readout is considerable. In many cases this inhibits CCDs from being used for fast dynamic multi-frame image accumulation.
Two examples of systems using CCDs are included in patent applications GB-A-2249430 and GB-A-2262383. Both applications are concerned with ways of overcoming the intrinsic CCD limitations.
Semiconductor pixel detectors comprise a semiconductor substrate with electrodes which apply a depletion voltage to each pixel and define a charge collection volume. Simple buffer circuits read out the electric signals when a photon is photo-absorbed or when ionizing radiation crosses the depletion zone of the semiconductor substrate. The buffer circuits can either be on the same substrate (compare EP-A-0,287,197) as the charge collection volumes or on a separate substrate (compare EP-A-0,571,135) that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique (bump-bonding is a technique known for a decade or more). These pixel detectors operate in a pulse mode. A pulse counting mode or simply pulse imaging can be implemented by either reading the pixels continuously or by reading pixels sequentially at a fast enough rate.
In either case, every time a charge is present as a result of a high energy ray or light, the aim is to read it out and process the information. The pixel detectors decrease the readout speed needed because there is a higher segmentation and more parallel readout channels. However, they cannot cope with high intensity applications because the readout electronics will overflow or counting ability saturates thus destroying the image contrast. In some of these devices simultaneously incident rays cause ambiguous and ‘ghost’ hits that cannot be resolved and worsen the resolution. Although these devices directly detect the incident radiation, they have limitations due to an operation based on a single pulse counting mode and imaging based on the counting of discrete points.
It will be appreciated from the above that all of the devices presently available have limitations which cannot be resolved. In particular CCDs enable charge from successive hits to be accumulated, but only to the limited extent possible within a potential well inside the Si substrate, which substantially limits the dynamic range. Also, because of the charge accumulation method, charge readout happens in a time sequence mode by clocking the pixel charge content to the neighbouring pixel storing unit (which is always the same Si substrate). Thus, until all pixels are read out as a time train sequence, a CCD cannot accumulate a new image frame since additional incoming radiation and/or light would not be recorded in one to one correspondence with a pixel position during the readout process. Therefore limited dynamic range and large inactive time during imaging are the two major CCD limitations.
On the other hand some semiconductor pixel devices have been proposed that directly read the pixel content every time a hit is detected. These devices operate on the single pulse counting mode and suffer from saturation problems at high counting rates. Such conventional single hit counting devices have a very small dynamic range.